This invention relates to transducers for sensing dynamic variations in strain in deformable structures such as cables or rods and for producing an indication proportional to the strain.
In many environments, particularly in connection with logging, construction and oil drilling activities, it is important to monitor the in-line or longitudinal strain due to the dynamic tensile and compressive forces acting on cables and rods in order to predict the safety of continued use of the cable or rod and its anchor. At one time, it was the custom to insert into a cable at some convenient point a device to be deformed by stress on the cable for directly measuring the strain. That is, the measuring or indicating device was included in series in the cable line. This meant that it was necessary to interrupt the cable in some fashion. The devices used had to be capable of withstanding the maximum expected load. They thus tended to be bulky, expensive and inaccurate. More recently, there has been a trend toward developing techniques for measuring loads on a cable which do not require interruption of the cable.
Austrian Pat. No. 174,967, issued May 26, 1953, discloses a device for measuring strain in a cable wherein the device is clamped to two spaced-apart points on the cable. All of the load is bypassed from the cable to load bearing lines in the device. A gauge for displaying the measured strain or tension is included in one of the load bearing lines. The use of such a device tends to degrade the safety margin below that which is inherent in the unmodified cable. That is because such a strain measuring device disturbs the normally uniform stress distribution in a cable. Disturbing the stress distribution in this manner tends to cause concentrations of stress at certain points. Where the stress concentration is excessively high, failure of the cable, sensor or both may occur. Additionally, if the load bearing lines were to rupture or break, a hazardous transient force would be applied to the cable. As another disadvantage of this device, problems may occur as a result of its use to measure strain in ordinary twisted cables. Such cables tend to rotate when they are stretched. Due to the fact that this device totally relaxes the cable in the region where the measuring device is attached, all of the rotation due to stretching of the cable will appear across the strain measuring device. This rotation tends to twist the device out of shape and also tends to impair its accuracy.
Seabury, Jr., U.S. Pat. No. 3,850,035 issued Nov. 26, 1974, typifies known devices for directly measuring cable line tension which use electrical or mechanical transducers. The cable bears against a cleat having limited and restrained rotatability. Rotation of the cleat is mechanically linked to a variable electrical resistor. Such devices are limited to sensing tensile loads only because their output signals are scalars. That is, the output signals do not change polarity when the load on the cable changes from longitudinal or axial tension to longitudinal or axial compression. Thus, the occurrence of compression loading cannot be distinguished from tension loading. In addition, the practical dynamic response of such devices is limited to low frequency strain variations. That is because the internal linkages of the devices have a resonance. The effects of dynamic strain variations at frequencies higher than the resonance frequency will be attenuated. Use of the device of Seabury, Jr., suffers from two additional disadvantages. First, the placement of the cleat is restricted to be at the end of the load-bearing portion of the cable. Thus, the strain-measuring device cannot be placed at any convenient point along the length of the cable. Second, the cleat must be placed in-line between the cable and an anchor for the cable. This requirement causes the same undesirable consequences to the system as devices which interrupt the cable as discussed above. As has been stated, such devices tend to be bulky, expensive and inaccurate. In addition, they tend to insert an additional impedance mismatch in the load-bearing line between a cable and its anchor. This alters the frequency characteristics of the system in a way which tends to complicate the task of measuring dynamic variations in the loading.
Miley, U.S. Pat. No. 3,871,217 issued Mar. 18, 1975, and Conoval, U.S. Pat. No. 4,158,962 issued June 26, 1979, are exemplary of devices which indirectly monitor the load on a rod or cable by inductively or optically monitoring the variation in the natural frequency of one of the many lateral or radial oscillatory modes of the deformable structure. Such devices are suitable for measuring static loads. However, it is difficult and expensive to attempt to use such devices to detect and measure dynamic axial or longitudinal loads on the cable or rod. That is because the transformation of the dynamic variation in axial strain to a variation in oscillatory motion is a complicated nonlinear process. Therefore, some of the dynamic axial strain information is likely to be lost. Furthermore, such axial strain information as does appear in the lateral oscillatory motion appears as modulation of a mode. Complex and expensive signal detection and demodulation schemes are thus required to extract the dynamic axial loading information from the lateral displacement oscillatory signal.
Siefert, U.S. Pat. No. 3,662,596 issued May 16, 1972, discloses a strain gauge device which measures strain in metal cords embedded within a tire; individual strain gauges are interconnected in a bridge. Such a device has the advantage of being capable of measuring both compressive and tensile dynamic loads. This device is difficult to attach to the cord being observed inasmuch as soldering or the equivalent is required. In addition, the attachment of such a device to cords or cables modifies the frequency characteristic of the deformable structure in a complex way since an impedance discontinuity is thereby introduced into the structure. As a result, stress concentrations occur at the discontinuities. The stress concentrations act as energy reflection points. This greatly complicates the task of predicting the consequences of the dynamic load variations acting on the cable and its anchor. In addition, the occurrence of stress concentrations increases the risk of mechanical failure of the cable itself.